// NOTE: 这里的snGrad计算存在bug
// SOLVED: tmp pointer
// const vectorField& USnGrad = U.boundaryField()[patchI].snGrad();


const tmp<vectorField>& tUSnGrad = U.boundaryField()[patchI].snGrad();
const vectorField& USnGrad = tUSnGrad(); // or use tUSnGrad.ref()

forAll(USnGrad, faceI)
{
    Info << "USnGrad[" << faceI << "] = " << USnGrad[faceI] << endl;
}
